This invention relates to liquid treatment plants and methods for their operation.
It is known to use a variety of plants for two-phase gasliquid mass transfer processes, for example in the biological clarification of waste water by the introduction of atmospheric oxygen.
One type of plant employs rotating impellers whose suction parts are installed just below the liquid surface, and which increase the interphase area by creating drops of liquid which are impelled through the gas phase. Other types of plant create bubbles of gas in the liquid, and increase the interphase area by pumping a compressed gas into the liquid through various types of diffusor (e.g. baffled columns etc., porous tiles, perforated pipes etc.).
The performance of the mass transfer process is controlled in the main by two parameters, others being equal: residence time and interphase area.
With the compressed gas processes the residence time can be increased by increasing the depth of immersion of the diffusor in the liquid. The interphase area can be increased by decreasing the size of the pores or holes in the diffusors. Either expedient, however, increases the power consumption in greater proportion than the rate of mass transfer, therefore the efficiency of the process is not improved. A further disadvantage of decreasing the size of the pores or holes in the diffusors is the increased danger of blocking by foreign bodies.
It is an object of the invention to provide an apparatus of simple construction, which increases the residence time and the interphase area in greater proportion than the increase in power consumption.
According to one aspect of the invention a liquid treatment plant for a two-phase, gas-liquid mass transfer process comprises a tank for containing a liquid so as to provide a gas volume above the liquid surface, a circulating pump for withdrawing liquid from said tank, a liquid injector pump downstream of said circulating pump, said injector pump having a nozzle opening through which liquid from said circulating pump can pass, an inlet through which gas can be drawn as a result of passage of liquid through said nozzle opening, and an outlet spaced axially from said nozzle opening, a mixing pipe extending from said outlet to a position adjacent the bottom of said tank, and a deflector for urging a gas-liquid stream within said mixing pipe in a direction transverse to the axis thereof, the cross-sectional area of a portion of said injector pump outlet adjacent said nozzle opening being between 1.05 and 1.4 times the cross-sectional area of said nozzle opening, and the cross-sectional area of said mixing pipe being between 4 and 11 times the cross-sectional area of said nozzle opening.
According to another aspect of the invention a method of operating a plant as above defined comprises maintaining the delivery head of said delivery pump at between 17% and 30% of the depth of the bottom of said mixing pipe below the surface of a liquid in the tank.
It is to be understood that the term delivery head relates to the heigh to which the circulating pump will deliver a liquid.
According to a further aspect of the invention a method of operating a plant as above defined comprises maintaining the delivery head of said circulating pump at between 0.7 and 1.4 meters of the liquid being pumped.
According to yet another aspect of the invention a method of operating a plant as above defined comprises maintaining a liquid level in said tank such that said level is not more than 10 diameters of said nozzle opening above, or more than 20 diameters of said nozzle opening below, the centre of said nozzle opening.
An example of the invention will now be described with reference to the accompanying drawings, in which: